docs: def npd_to_pmf(nodal_plane_dist, use_default=False):
      """
      Returns the nodal plane distribution as an instance of the PMF class
      """
      if isinstance(nodal_plane_dist, PMF):
          # Aready in PMF format - return
          return nodal_plane_dist
      else:
          if use_default:
              return PMF([(1.0, NodalPlane(0.0, 90.0, 0.0))])
          else:
              raise ValueError('Nodal Plane distribution not defined')